Scientific Forecaster Discussion

Synopsis...the strong and persistent upper ridge will retreat off the West Coast over the next couple of days, allowing weather disturbances to begin eroding the strong valley inversions, with the strongest of these storms expected on Saturday.

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Short term (through 00z sunday)...a shortwave trough is dropping into Utah and southwest Wyoming from the north this afternoon, bringing increased cloud cover and some snow showers. Most of this precipitation is not reaching the valley floors, but light accumulating snowfall is possible in the higher terrain. The best moisture and instability with this disturbance is expected to remain to the east of the forecast area as the wave passes tonight.

Though this system has begun to weaken low level inversions across northern Utah, it should not be strong enough to really mix the valleys. Many locations have had some dissipation of fog and stratus through today, but it is likely that some of the low level clouds re-form after midnight.

The other primary expected impact of this shortwave trough and the associated cold front is increased winds across the southern half of Utah, especially in north-northwesterly downslope and canyon areas. Have increased winds tonight across Castle country, and even moreso in Washington County. Tonight looks like a particularly good set-up for north-northeasterly Washington County gap winds, with a 7 mb mslp gradient from Cedar City to Las Vegas, 700mb flow of 40-50 knots, and good cold advection. As such, have issued a High Wind Warning on this shift, with winds expected to peak during the early morning hours Thursday.

After the system exits to the south, high pressure re-builds over the region through Thursday and Friday, with inversions strengthening again. The next shortwave trough is still forecast to push into the intermountain west on Friday night and Saturday, though there is some uncertainty about how much the storm will split before it reaches Utah. While a more splitty system would be less efficient at scouring out the valleys, even the less aggressive GFS solution would still dissipate the inversion fairly effectively, with increased cloud cover, a chance of snowfall, and 700mb temperatures eventually falling to about -12 celsius by Saturday evening. Hopefully the models will return to better agreement about the details soon, but at least some light accumulation of snow is likely in the northern mountains, with a chance of showers in the valleys as well through the day Saturday.

Long term (after 00z sunday)...regardless of the amplitude of the trough overhead heading into Saturday night, models continue to remain consistent in downstream propagation and decreasing precip potential (very light snowfall) into Sunday morning.

In wake of the trough passage low amplitude ridging will build into the region, with a somewhat moist anticyclonic northwest flow aloft clipping the Utah/Idaho border region early next week. Outside of periodic cloud cover in the north, do not foresee much precip potential as the jet will remain well north of the area.

Of more note is the developing long wave pattern across the western Continental U.S. Beginning later next week. Global ensemble mean progs are in agreement that retrogression of the Pacific Ridge will become more notable, this potentially allowing for a higher likelihood of short wave energy rolling over the ridge and impacting the intermountain region. Deterministic output in the globals suggest this, but not buying off on their details at this point (low confidence in short wave evolution timing attm). It's more of a big picture idea currently, and the good news points towards a minimal threat of any redevelopment of strong inversions, and the potential for more frequent precip (even if on the light side with clipper like systems). Time will tell.

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Aviation...redevelopment of low stratus and the potential of dense fog will continue to be the primary operational weather concern at kslc this taf period. Prevailing IFR conditions are expected through tonight in haze/fog, with redevelopment of stratus in the 005-010 range. It's anticipated that the potential for dense fog formation will hold off until after 07z, then a 60% chance exists for LIFR/vlifr fog into tomorrow morning.

Light snow fall out remains a potential at times overnight into tomorrow morning, with any fall out creating light accumulations on untreated surfaces. Light winds will prevail.